The present invention relates to an exhaust gas recirculation control system, and, more particularly, relates to an exhaust gas recirculation control system which is particularly suited for application to a diesel engine.
The exhaust gas recirculation in diesel engines is to replace a part of the air inhaled into the cylinders of the engine that is generally in excess of that which is required for combustion of the fuel injected into the cylinders of the engine, by recirculated exhaust gas, in order to suppress emission of harmful NOx pollutants. In this diesel engine exhaust gas recirculation, it is desirable that the amount of air which is replaced by exhaust gas should be proportional to the amount of air which is in excess of the actual air requirement for combustion of the actual amount of fuel injected, so that maximum possible amount of excess air is reduced from the air/gas flow supplied to the cylinders of the engine, without causing unstable combustion of fuel in the cylinders, while accomplishing maximum effect of suppressing NOx emission, over the entire operational region of the engine. Since the excess air ratio in the diesel engine decreases as the load on the engine increases, it is necessary to control the exhaust gas recirculation quantity so that the exhaust gas recirculation ratio decreases, as the load on the engine increases. The exhaust gas recirculation ratio is defined as the ratio of the quantity of exhaust gas recirculated and introduced into the inlet system of the engine to the total quantity of inlet gases inhaled by the engine, which is the sum of the quantity of the recirculated exhaust gases and the amount of fresh air inhaled by the engine.
Therefore, in order to control exhaust gas recirculation in a diesel engine, it is necessary to obtain a control signal which represents engine power output or load, and is effective for operating a means for controlling exhaust gas recirculation. In the case of a gasoline engine, wherein the speed regulator is the inlet throttle valve which controls the amount of intake mixture, the inlet manifold vacuum can be effectively used as such a control signal. However, in the case of a diesel engine, the power output of or the load on the engine is controlled by the amount of fuel injected per unit time, and, therefore, it is not generally possible, with a diesel engine, to perform control of exhaust gas recirculation, according to the load on the engine, by using a diaphragm operated type exhaust gas recirculation control valve which responds to inlet manifold vacuum, as is done commonly with gasoline engines. Accordingly, therefore, in the prior art, in a diesel engine the conventional exhaust gas recirculation control valve has been directly connected to and operated by either the accelerator pedal linkage of the vehicle, or the control lever of the fuel injection pump, so that the exhaust gas recirculation control valve has been operated according to the operation of the accelerator lever, or the control lever.
This form of control means for exhaust gas recirculation is fairly easy and simple to manufacture, but there is a problem that it tends to increase the amount of force required for manipulation of the accelerator pedal, and thereby may deteriorate the operational feeling of the accelerator pedal and therefore the drivability of the vehicle.
As an alternative system for controlling diesel exhaust gas recirculation, there has been proposed a system which comprises a diaphragm type exhaust gas recirculation control valve, the diaphragm device being actuated by vacuum provided by a pneumatic governor diaphragm chamber installed in the fuel injection pump. However, with this system of exhaust gas recirculation control, the problem arises that it is not really possible to obtain enough power for operating the exhaust gas recirculation control valve from the vacuum supplied by the penumatic governor diaphragm chamber, because the vacuum present in this pneumatic governor diaphragm chamber is basically relatively small. Further, because of this, there arises the problem that the position of the exhaust gas recirculation control valve may be directly displaced by the dynamic pressure of the inlet air flow and/or the recirculating exhaust gas flow.
As another possible solution to the problem of diesel exhaust gas recirculation control, the possibility has been explored of controlling exhaust gas recirculation quantity continuously to the appropriate and correct value by measuring the amount of fuel injected to the combustion chambers of the engine per one cycle, and by opening and closing an exhaust gas recirculation control valve by a pressure type and/or electric type actuator, based upon these measurements. However, in this case, the control system as a whole becomes very complicated, and various problems occur when it is in practice mounted to an operating automobile.